Generally, it has been known that, in a loudspeaker system, the harmonic distortion at lower frequencies results from the non-linearity of the enclosure spider and like elements involved in the mechanical system and that the harmonic distortion at higher frequencies is due to Bl distortion and current distortion. In order to reduce the latter distortion, that is, the Bl distortion and current distortion, various approaches have conventionally been proposed including the following typical measures:
(i) A member composed of a material having a low hysteresis, for example a laminated core, is arranged near an air-gap defined by the pole piece and the plate of the loudspeaker is selected. This improves the non-linearity of the voice coil impedance and reduces the current distortion.
(ii) The pole piece is capped with a copper cylinder to cancel the magnetic flux generated by the voice coil.
(iii) A copper ring or similar member is placed so that the magnetic flux of the magnetic circuit passes through the ring in order to cancel the magnetic flux generated by the voice coil.
However, these conventional measures for reducing the harmonic distortion are, so to speak, passive ones in which their effect in reducing the distortion is restricted by the magnitude of the conductivity and hysteresis of the material and are disadvantageous in that the reduction of harmonic distortion cannot be attained at will. Turning to a conventional moving coil type velocity sensor, it has a driving assembly which consists of a magnetic circuit, a velocity sensing coil and a spider, and therefore resembles the driving assembly of the electro-dynamic loudspeaker. Consequently, it suffers from harmonic distortion which is similar to that exhibited by the electro-dynamic loudspeaker. The harmonic distortion exhibited by the moving coil type velocity sensor mainly results from the fact that the magnetic flux density across the velocity sensing coil corresponding to the voice coil of the loudspeaker changes as the velocity sensing coil moves. In order to eliminate the change in the magnetic flux density, it is necessary to increase the width of the velocity sensing coil or make the thickness of the plate larger than the width of the coil, resulting in the additional disadvantage that the size of the magnet is increased together with an increase in the size and weight of the sensor itself.